The present invention is related to optico-thermal magnetic storage media used in magneto optic memories, magnetic records, and display elements.
Presently known optico-thermal magnetic storage media are polycrystal films of MnBe, MnCuBi and the like, amorphous films of GdCo, GdFe, TbFe, DyFe, GdTbFe, TbDyFe, and the like and monocrystal films of G1G and the like. The above mentioned amorphous films are at present considered superior because of their capability of being formed over a wide area at about room temperatures, high efficiency in writing due to the smallness of the optico-thermal energy required for writing, and the superior efficiency of reading the written signals with a high signal-to-noise ratio.
However, these amorphous films also display a number of weaknesses. For example, GdFe has a small retentivity or coercive force and the recorded information is unstable. With GdFe and GdCo the magnetic compensation point is used for writing, and the film composition must be very carefully or strictly controlled during manufacturing to achieve uniform writing efficiency. With TbRe, DyFe, and TbDyFe, writing is performed by utilizing the Curie point, and the film composition need not be controlled so carefully, but the Curie point is only about 100.degree. C. and a powerful light source cannot be used for reading the signals. Writing efficiency improves with lower Curie temperatures, but the written signals will be disturbed by the ambient temperature or by the light for reading. As a result, the Curie temperature should be as high as the writing is possible, but considering practical limitations a temperature of about 200.degree. C. is optimum.
The signal-to-noise ratio of the reflected ray at readout is proportional to .sqroot.R .theta..sub.k, where R is the reflection coefficient, and .theta..sub.k the Kerr rotation angle. This relation shows that a larger Kerr rotation angle is advantageous to provide a good signal-to-noise ratio for the readout.
Table 1 at the end of the specification shows the Kerr rotation angle and curie temperatures for films of the main amorphous optico-thermal magnetic storage media.
Japanese Patent Application Laying-open No. 153546/1981 adopts a method for producing films with high Curie temperatures. It proposes a medium that is composed of a writing layer of high retentivity and vertical magnetic anisotropy on the surface of the substrate, and has on top of this writing layer a readout layer that has low coercive force or retentivity and vertical magnetic anisotropic properties.
As shown in Table 1 the TbFeCo film has an advantageous Curie temperature of around 200.degree. C., and the largest Kerr rotation angle of the compounds, and gives a good signal-to-noise ratio that is advantageous for readout. However, the Kerr rotation angle is dependent on the Tb content, and Tb content should be low to achieve larger Kerr rotation angle. This may be seen from FIGS. 1A and 1B.
FIGS. 1A and 1B show the characteristic changes in Kerr rotation angle (.degree.) and Curie temperature (.degree.C.) for various Tb.sub.x (Fe.sub.1-y Co.sub.y).sub.1-x compositions.
In the figures, (A) shows a characteristic for y=0.7, and (B) for y=0.1. In FIG. 1A the abscissa is the composition (x) and the ordinate the Kerr rotation angle (.degree.) while in FIG. 1B the abscissa is the composition (x) and the ordinate is the Curie temperature (.degree.C.). When Tb content is low, noise level increases due to temperature dependence of saturation magnetization and signal-to-noise ratio is lowered. As a result, Tb content of 22-35% in the TbFeCo film is optimal. There is also the disadvantage that the Kerr rotation angle increases with Co content, increasing the Curie temperature and making higher energy necessary for writing.
With TbFeCo ternary alloy films of the composition Tb.sub.x (Fe.sub.1-y Co.sub.y).sub.1-x, the Curie temperature of the film at y.gtoreq.0.5 becomes too high for either the first layer or the second layer, making recording impossible. This is also the case with DyFeCo films. With DyFeCo films it is possible to obtain Curie temperatures around 200 C. and large Kerr rotation angles, giving good signal-to-noise ratios for readout. The composition of DyFeCo films determine the Kerr rotation angle and Curie temperature, and larger Kerr rotation angle require smaller amounts of Dy. This is shown in (b) of FIG. 2A and FIG. 2B. FIGS. 2A and 2B are for Dy.sub.x (Fe.sub.1-y Co.sub.y).sub.1-x films and FIG. 2A shows characteristic changes in composition (x) and Kerr rotation angle and FIG. 2B shows changes in composition (x) and Curie temperature (.degree.C.).
In the figures, (A) is the characteristics for y=0.20 and (B) for y=0.1. In FIG. 2A the abscissa is the composition (x) and the ordinate is the Kerr rotation angle (.degree.), while in FIG. 2B the abscissa is the composition (x) and the ordinate is the Curie temperature (.degree.C.). With low Dy contents the noise level increases due to the temperature dependence of the saturation magnetization (Ms). Due to this noise level increase, the signal-to-noise ratio is lowered. Accordingly, Dy contents of 22-35% are optimum in DyFeCo films. With increasing Co content the Kerr rotation angle increases but the Curie temperature rises, making a high energy necessary for writing.
Use of DyFeCo ternary alloy of the Dy.sub.x (Fe.sub.1-y Co.sub.y).sub.1-x composition, for either the first or second layer with y.gtoreq.0.5 will give too high Curie temperatures of the film, making recording impossible.
To utilize the method shown in Japanese Patent Application Laying-open No. 153546/1981 which comprises writing information in the writing layer, transferring the information from the writing layer to the reading layer and reproducing the information from the reading layer by irradiating polarized light into the reading layer, it is required that the combination of the layers composition must be selected to obtain good transfer to achieve this, TbFe film, DyFe film or the like should be used for the writing layer, and GdFe films, GdCo films or the like should be used for the reading layer. This leads to a complex structure and a complex manufacturing process. There is a further problem in that the writing layer is between the reading layer and the substrate, and the light for regeneration does not pass through the substrate. Even with the mediation of a protective film, readout is directly from the film side, and so may be affected by dust and other debris on the medium.
In the Japanese Patent Application Laying-open No. 78652/1982, a layer capable of vertical magnetization and having strong retentivity, vertical magnetization and a low Curie point is formed on one surface and a layer with weak retentivity and high Curie point is formed on the other surface. This high retentivity layer and the low retentivity layer are exchange-coupled.
In the above Japanese Patent Application Laying-open No. 78652/1962, films of TbFe and DyFe are mentioned as examples of high retentivity layers and films of GdFe and GdCo as low retentivity layers. However, two-layer films with different elements is complicated in structure and the manufacturing conditions are difficult to establish.
Even media that are exchange-coupled may not be satisfactory. For instance, with a combination of TbFe film with magnetic dominance of the Tb sub-lattice and GdFe film with magnetic dominance of the Gd sub-lattice, a large Kerr rotation angle cannot be obtained and improvement in the signal-to-noise ratio cannot be expected. With magnetic dominance of the Fe sub-lattice in TbFe films and magnetic dominance of the Fe sub-lattice in GdFe films the noise level increase and improvement in the signal-to-noise ratio cannot be expected.